Hydrogen cloud of a comet

Fig. 14.12 . A comparison of the visible image (left) of comet Kohoutek with a far ultraviolet image (right) on the same scale, taken from Aerobee rocket flights on 4 and 7 January 1974. The ultraviolet image shows a gigantic cloud of hydrogen nearly 10 million, or 107, kilometers in size, or eight times bigger than the Sun. It is being fed by the comet nucleus at the rate of 500 billion billion billion, or 5 x 1029, atoms of hydrogen every second. The large size of the hydrogen cloud is due to the fact that hydrogen atoms are much lighter, and move into space faster, than the other atoms, ions, molecules and dust particles, which produce the visible light of the coma. (Courtesy of Chet B. Opal, NRL.)